1. Field of the Invention
The present invention relates to the production of propylene polymers. In particular, the present invention concerns film-making terpolymers of propylene and at least two alpha-olefin monomers and a process for producing the polymers. The present terpolymers are suitable, e.g., for applications where good heat sealability and softness are required.
2. Description of Related Art
Polyolefins used in film applications as sealing layers should have low melting temperatures for good heat sealing performance. In polypropylene films, random copolymers having a relatively high content of comonomers, usually ethylene, are commonly used as such a sealing component. A high comonomer content is required for obtaining low melting temperatures. There are, however, problems associated with the production of random copolymers as well as with the end-properties of the material. This is on account of its low melting temperature, broadness of melting range due to poor comonomer distribution, and polymer solubility in the polymerisation medium. By incorporating a third monomer, such as 1-butene or another higher alpha-olefin, the above-mentioned problems can, to some extent, be overcome, and a number of processes for producing terpolymers of propylene are known in the art. However, there are still problems relating to the production of the polymers and to the level of solubles of the products.
In comparison to homopolymer production, when a terpolymer or another high comonomer content polymer having a low melting temperature is to be produced in a slurry reactor, the reaction temperature in that reactor must be lowered because of the high solubles content in the polymer. Further, the comonomers used in the polymerization, such as ethylene and butylene, cause swelling of the polymers in the polymerization medium of the slurry reactor. When swollen and soft polymer particles are flashed after polymerization, the morphology of the particles is destroyed and the bulk density of the powder becomes very low. There is also a higher demand for external heat before it is possible to evaporate unreacted monomer after the slurry reactor. Further, the surface temperature of the flash line must be lowered because of the low melting temperature of the product. Liquid monomer entering the flash together with a sticky polymer will plug the vessel. If pressure in the flash vessel is reduced too much to improve monomer evaporation, flashing takes place too fast and, as mentioned above, the morphology of particles is destroyed causing problems in powder handling.
The above-mentioned problems are aggrevated when the proportion of comonomers in the terpolymer increases.
For this reason, in the prior art polymerization of terpolymers having a melting temperature below 132xc2x0 C. has been carried out by means of gas phase processes.
In EP 0 674 991 a propylene terpolymer is disclosed which comprises 20 to 60 wt-% of a copolymer of propylene and ethylene, containing 1 to 5 wt-% of ethylene, and 40 to 80 wt-% of a copolymer of propylene together with ethylene and a C4-C8 alpha-olefin, the ethylene content being 1 to 5 wt-% and C4-C8 alpha-olefin content being 6 to 15 wt-%. Said product is produced preferably in two gas phase reactors. The C4-C8 alpha-olefin is added in the second gas phase reactor.
U.S. Pat. No. 4,740,551 discloses a process for manufacturing propylene-ethylene impact copolymers. According to this known process, propylene is first homopolymerized in a plug flow pipeline reactor, the polymer is then transferred to a second step, wherein homopolymerization is continued by adding propylene, whereinafter the polymer mixture is transferred to a third gas phase step and polymerization is carried out in the presence of propylene and ethylene for producing said impact copolymers. In the above mentioned patent no other alpha-olefin is used in the third step, which is the only step in which a comonomer other than propylene is used.
It is an object of the present invention to provide novel materials based on terpolymers, which can be used for manufacturing films having good heat sealing properties.
It is another object of the present invention to provide a process for producing terpolymers of propylene, ethylene and other alpha-olefins in a process comprising in particular slurry and gas phase reactor(s) connected directly together, while avoiding the disadvantages related to operability when producing high comonomer content products.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the finding that the properties of propylene terpolymers can be improved by increasing the amount of heavier comonomers in relation to ethylene. It has further been found that such terpolymers are advantageously produced by means of multi-reactor polymerisation technology. Thus, the invention comprises using a combination of two or more reactors preferably connected in a cascade for producing a mixture of different propylene terpolymer compositions, so as to obtain a polymer product exhibiting a ratio of ethylene-to-butylene (or heavier alpha-olefins) of less than 0.3. Such comonomer distribution (randomness) gives a material having low solubles content of preferably less than 6.5% hexane solubles (determined by FDA test), good optical properties and good processibility.
According to the present invention, it is particularly preferred to produce the material in a combination of one or more bulk polymerisation reactor(s) and one or more gas phase reactor(s). Thereby, the comonomer conversion can be increased, operation of the copolymerisation process is facilitated by using the gas phase, and the structure and properties of the resulting product lead to improved heat sealability of articles manufactured from the product.
According to the process as described herein, a process has been designed for producing high comonomer content polymers having a low melting point in at least one slurry reactor in the presence of catalysts at elevated temperature. The process comprises:
a) feeding into a slurry reactor a reaction mixture containing 50 to 85 wt- % of propylene, 1 to 10 wt-% of ethylene, 15 to 40 wt-% of another alpha-olefin, a catalyst system capable of achieving olefin polymerization at said temperature conditions, and optionally hydrogen,
b) polymerizing said reaction mixture at a temperature of less than 70xc2x0 C. for a sufficient time to obtain a propylene terpolymer amounting to 50 to 99 wt-% of the end product,
c) transferring said reaction mixture into a gas phase reactor operating at a pressure higher than 5 bar, preferably higher than 10 bar, and optionally adding 0 to 10 wt-% of ethylene, 0 to 10 wt-% of another alpha-olefin, 0 to 40 wt-% of propylene, of said transferred mixture, and optionally hydrogen, and
d) continuing polymerization in said gas phase reactor for obtaining a propylene terpolymer amounting to 1 to 50 wt-% of the end product, whereby a terpolymer is obtained having a melting temperature of less than 135xc2x0 C., preferably less than 132xc2x0 C.
Thus, according to the present invention, the terpolymerization is carried out in the slurry phase, preferably in a loop reactor by using relatively high amounts of C4-C8 alphaolefins as comonomers. This is possible because the polymer slurry is transferred directly into a gas phase reactor without separating the reaction medium.
According to one embodiment a loop reactor is used as said slurry reactor. According to another embodiment said slurry phase is carried out in two slurry reactors, preferably but not necessarily in two loop reactors. In this way the comonomer distribution can be easily controlled. When continuing the polymerization in a gas phase reactor or reactors, comonomer content can be increased further. Thus, the final polymer properties can be tailored by adjusting comonomer ratios in different reactors.
Next, the invention will be examined more closely with the aid of a detailed description and with reference to the attached drawings and the following working examples.